Materiales híbridos basados en nanocarbones y polioxometalatos para aplicación como electrodos en supercondensadores con mecanismo dual de almacenamiento de energía

Abstract

Batteries and supercapacitors are two types of energy storage devices with complementary working mechanisms and performances. Batteries feature a storage mechanism based on faradaic redox reactions and present high energy densities but low power densities. On the other hand, the electrostatic mechanism of double-layer supercapacitors provides high power densities but low energy densities. The design of hybrid electrode materials combining both mechanisms could be a practical way to improve simultaneously energy and power in one. In this work we have explored the synthesis, properties and application of hybrid materials based on three types of nanocarbons (Activated Carbon, AC; Multiwalled Carbon nanotubes, MWNTs; and Reduced Graphene Oxide, RGO) with three electroactive polyoxometalates (POMs) with the well-known Keggin structure (phosphomolybdic acid H3PMo12O40, phosphotungstic acid H3PW12O40 and silicotungstic acid H4SiW12O40). Carbons provide a conducting matrix with capacitive energy storage capabilities (double-layer) whereas the inorganic clusters provide their molecular faradaic electroactivity. Our work has confirmed our initial working hypothesis, namely, that it was possible to synthesize all different hybrid combinations, leading to materials in which polyoxometalates are permanently integrated in each of the carbon matrices. The incorporation of POMs takes place though through different mechanisms and reaching maximum POMs concentration which are different for each carbon, following the order AC > RGO > MWNTs. Thus, in AC matrices, POMs are bound by physisorption processes in suitably sized micropores, whereas in RGO, POMs are anchored by chemical interactions with polar oxygenated groups present in the carbon. All the hybrid materials prepared were characterized by TGA, XRD, SEM, TEM, adsorption isotherms, spectroscopies (FTIR, XPS, Raman for RGO materials), as well as with systematic electrochemical studies (CV, charge-discharge cronopotentiometry) on threeelectrode and two-electrode cells (symmetrical supercapacitor devices). Most of the materials studied led to electrodes with a synergic combination of properties, including improved energy and power densities per unit volume in comparison with the parent nanocarbons, with an enhanced voltage range in the case of the polyoxotungstates and able to stand repeated charge-discharge cycles of up to tens of thousands (104 ) of cycles, much improved with respect to the pristine carbon matrices under the same cycling conditions. Thus, our work has shown a way towards improving energy storage properties through the hybridization of electrode materials combining faradaic and capacitive components. We have shown the possibilities of this approach using polyoxometalates and nanocarbons as case examples that in fact provide a double synergy, by combining faradaic and capacitive storage and by providing a conducting matrix (carbons) which makes possible the harnessing of the electroactivity of these molecular clusters.

Date of Award	2020
Original language	Spanish
Awarding Institution	Universitat Autònoma de Barcelona (UAB)
Supervisor	Pedro Gómez Romero (Director) & Vanesa Ruiz Ruiz (Director)